A fixing device includes a heating body, a pressuring body, a heat source and a detecting mechanism. The detecting mechanism is not in contact with the heating body and including an infrared detecting element which detects infrared rays radiated from an outer circumferential face of the heating body. A longitudinal direction of the heating body is a second direction which crosses a first direction as a conveying direction of a recording medium. A detected area is arranged on the outer circumferential face of the heating body so that the infrared rays radiated from the detected area is detected by the infrared detecting element. The detecting mechanism is arranged in a posture inclined to another posture facing the outer circumferential face of the heating body so that a width in the second direction of the detected area is wider than a width in the first direction of the detected area.